Over the years, many suggestions have been made for joining circular air ducts. The following U.S. Patents suggest various couplings for circular or rectangular pipe:
______________________________________ 180,416 3,689,114 1,921,642 4,447,078 1,762,766 4,558,892 1,811,277 4,669,762 3,415,543 4,941,693 ______________________________________
Despite these coupling designs, the principal way in which circular ducts have been connected over the years is the use of the so-called double S-lock. Such is simply a strip of sheet metal that has been folded upon itself to provide oppositely opening grooves and then bent into circular configuration with the ends aligned and joined in any suitable fashion. Such double S-lock locks have been used for both circular and rectangular ducts. After they are installed, drive screws are inserted every 3" or so around the duct and then the joint is wrapped and painted to effect an airtight seal. While the double S locking joint is in itself inexpensive, the time required by the duct installer raises the cost per joint considerably. Effecting airtight joints in circular duct work has been quite labor intensive.
In U.S. Pat. No. 4,941,693, a connector is disclosed which is formed of two cylindrical shapes that must be deformed and thereafter nested and riveted together. The cost of manufacture of this coupling makes it expensive to use. In addition, because of the relatively long axial depth of the oppositely opening grooves in relation to their radial width, when sealant is placed in the grooves and it is then attempted to insert the ends of the ducts, a hydraulic lock tends to develop making it difficult to obtain satisfactory insertion of the duct ends in the grooves. Additionally, as the ducts are slid over the projecting flanges of the coupling, the sealant tends to be wiped off the flanges thereby destroying the seal between the duct and the coupling at such flanges. Clow U.S. Pat. No. 180,416 is similar to the structure of U.S. Pat. No. 4,941,693 and presents many of the same problems in its use.
U.S. Pat. No. 4,669,762 if somehow usable for circular duct work would create a hydraulic lock between the sealant, the duct and the coupling as the duct is inserted in the coupling.
My own prior U.S. Pat. No. 3,415,543 was never adapted for use with circular ducts and the relatively large channel section could not be bent into circular configuration.